The Preparation Of MnO₂-C Composite Powder For Supercapacitor And Performance Optimization For Capacitor Devices

MnO₂ aqueous supercapacitor, with the advantages of low cost, low pollution and high theoretical specific capacity, turns out to be a promising field in energy-storage research. But the electrochemical performance of MnO₂ is ultimately limited by its poor electrical conductivity and its low initial specific surface area. In order to change the surface area of MnO₂, 5 kinds of MnO₂/Nano carbon sphere composite powders were prepared by different ways. Their crystal structures and morphologies were characterized by scan electron microscopy（FSEM）, X-ray diffraction（XRD） and N2 adsorption test（BET）. The electrochemical behavior were measured by cyclic voltammetry（CV）, galvanostatical charge/discharge（GCD） and electrochemical impedance spectra（EIS）. Furthermore, the relation between specific capacity and specific surface area of δ-MnO₂ was studied. The influence on the potential window of the electrodes mass ratio has also been discussed. The main results of this research are as follows:In 5 types MnO₂/Nano carbon sphere composite powders, the mass fraction of MnO₂ reached 50%, and the basic crystal structures were δ-MnO₂. The specific surface area and the porous distribution of these powders differ a lot, the largest specific area reached 170.2m2/g with the specific capacity reached 158.8F/g.Rate capability of the composite electrodes is closely related with the electrode powder pore diameter distribution. Among the as-prepared powders, two samples with the largest amount of macroporous had the worst rate capability, the decay of specific capacity at the scanning rate of 50mV/s reached 35.4% and 30.3% compared with the data at 5mV/s; and the samples contained more mesoporous had better rate capability, whose decay were merely 23.8% and 19.8%.In this work, the specific surface area of three groups of nano carbon spheres/δ-MnO₂ composite powder was greatly improved compared with single δ-MnO₂, which means composite process could effectively improve the utilization of MnO₂. The specific capacity and specific surface area of δ-MnO₂ with the pore distribution mainly consisted of mesoporous is a linear relation.The electrodes mass ratio R reached 0.5, realistic operating window was higher than the expected value, the color of electrolyte changed, which means irreversible reactions from Mn（IV） to Mn（VI） taken place. And the efficiency of capacitor diminished when R is higher than 2. The appropriate potential window of MnO₂-NCs//AC aqueous asymmetric supercapacitor in this work should between 1.6¹.8V. And the specific capacitance of the negative electrode is higher than that of the single electrode in the actual discharge process.The capacitor with a mass ratio of 1.5 in this work has the largest energy density, which reached 22.3 Wh/kg with a power density of 300W/kg. And the energy density could still maintain a relative high value（17.6 Wh/kg）, when the power density raised to 1300W/kg. The result revealed that device with R=1.5 was superior compared with that R=1.0, although 1.0 is closer to theoretical calculating value. Similar phenomen were obtained with R=0.5、0.7 and2.0. The energy density was lowest when R reaches 2.5, but its low resistance leads to the maximum power density.